1. Field of the Invention
Diamond is a material that has uniquely attractive physical, electrical, thermal, and optical properties. It is also a metastable crystal at room temperature that forms naturally only at extremely high pressures and temperatures. Small industrial diamonds have been grown artificially for a number of years using catalysis, while more recently chemical vapor deposition (CVD) processes have been used to grow relatively large diamond thin films that have a wide variety of mechanical and optical uses. All of these synthesis processes produce commercial diamond that is small in at least one dimension. The generic problem with current processes used for artificial diamond growth is that the growth rate is too slow to be practical for bulk diamond growth. There have always been applications for which bulk diamond would be a far superior choice to presently available materials as a result of the high strength, high thermal conductivity, high resistance to thermal shock, and infrared transmissivity of diamond. An additional requirement for a bulk diamond fabrication process is that it should be a net shape growth process, because machining and polishing an arbitrary piece to a final shape would be extremely difficult.
2. Description of the Related Art
It is the primary object of the present invention to provide a process by which separate diamond particles can be joined together to form a single piece of polycrystalline diamond.
Heretofore, synthetic diamond has been created using a variety of processes for growing crystals. All of these processes create crystals by steadily accreting new atomic matter on the surface of some small seed crystal or substrate. For diamond, the form of atomic carbon appropriate for surface accretion is only naturally available from condensed carbon at extremely high temperature and pressure. Catalytic processes reduce the necessary conditions dramatically but growth conditions are still difficult to achieve and growth rates are very low. CVD processes use plasma or combustion conditions to provide suitable carbon, but the low density of atoms associated with a gas or plasma again leads to low growth rates in one dimension, although wide areas of thin films can be grown.
Fullerenes are one form of pure carbon where the molecules that are made up of carbon atoms form a hollow shell. C.sub.60 is one such molecule that is unusually stable and has a spherical structure. Fullerenes complement the planar form of carbon (graphite) and the tetrahedral carbon crystal known as diamond. Fullerenes are different from graphite and diamond in that they can have a high vapor pressure at moderate temperatures and in that they have a high solubility in common solvents. Fullerenes are potential precursors in many plasma chemistry and chemical vapor deposition processes.
Gruen et. al. 1,2! have grown diamond thin films using C.sub.60 fullerene as the precursor in the standard plasma-enhanced chemical vapor deposition (PECVD) process. The extensive fragmentation of C.sub.60 to C.sub.2 in an argon plasma observed by Gruen et. al. led them to develop the use of C.sub.60 as a precursor for CVD growth of diamond films. Fullerene containing soot was placed in a sublimator oven, heated to 550 degrees C. (.degree.C.), and an Argon/Hydrogen (Ar/H.sub.2) mixture (20 standard cubic centimeters per minute--sccm Ar, 4 sccm H.sub.2) was passed through the sublimator into the plasma chamber while maintaining a total pressure of 100 torr. A 76 millimeters (mm) diameter single-crystal silicon disk, which had been mechanically treated with 0.1 mm diamond powder, was placed on a graphite holder and maintained at 850.degree. C. during the experiment. A 1500 Watt (W) microwave discharge was maintained in the gas mixture during a 16 hour (h) deposition. Optical emission measurements showed intense C.sub.2 Swan band emission, as well as intense hydrogen alpha (H.sub..alpha.) and H.sub..beta. but relatively much weaker Ar emission lines. The silicon substrate was examined after the deposition using scanning electron microscopy (SEM), x-ray diffraction (XRD), and Raman spectroscopic techniques to verify diamond deposition 2!. Under these conditions, no film growth was observed in a fullerene-free argon plasma. Deposition was carried out at 850.degree. C. with 1500W of microwave power for 1-3h. Based on cross-section scanning electron microscope (SEM) images, it was estimated that the diamond growth rate was about 1.2 mm/h, which is at least comparable to that observed using 1% methane in hydrogen as a precursor under similar deposition conditions.
In situ optical measurements reveal very intense C.sub.2 Swan band emission, which is believed to be a result of collisional and other fragmentation processes of the C.sub.60. 3! The C.sub.2 fragments are thought to be the primary precursor species for fullerene diamond growth. The Swan band emission is an experimental indication that C.sub.60 fragmentation occurs as a result of successive elimination of neutral C.sub.2 groups. 4!.
Fullerene Photofragmentation Many studies have shown that C.sub.60 photofragmentation results in even-numbered clusters in the range C.sub.60 -C.sub.32 e.g. 5!. With this approach, rate constants for fragmentation and for delayed electron emission have been deduced. Wurz and Lykke 6,7! have investigated the processes of fullerene photofragmentation and photoionization. The interaction of intense laser light in the visible and UV wavelength range with gas-phase C.sub.60 leads to high internal excitation (.about.50 electron-Volts (eV)) of the C.sub.60 molecule rather than direct multiphoton ionization. C.sub.60 has a very rigid and highly symmetric molecular structure that has a very high density of vibrational states and thus an extremely rapid thermalization (10.sup.-14 seconds (s)) of a photoexcited electronic state. For this reason the neutral C.sub.60 molecule has been shown experimentally and theoretically to absorb 10-20 photons to form a superexcited molecule, even though 2-3 photons are sufficient to achieve multiphoton ionization (7.6 eV ionization energy). Delayed ionization (time scale 1-10 milliseconds) and fragmentation both have these high internal excitations as common precursors and both were found to occur at about the same rates.
The experimental work of Wurz and Lykke investigated a wide fluence and wavelength range to map out the different parameters that characterize these processes. A Tantalum cup filled with approximately 200 milligrams of pure C.sub.60 was held at a temperature of 800 Kelvins to give a vapor pressure of about 5.times.10.sup.-4 Torr. Laser fluences from 0.01 to 10.sup.3 millijoules per square centimeter were used at wavelengths of 212, 266, 355, and 532 nanometers (nm). As long as the photon energy was less than that needed for direct ionization, high fluence laser irradiation of C.sub.60 resulted in thermal fragmentation and delayed ionization with equal efficiencies. Thus, for submicrosecond laser pulses photofragmentation is the dominant result of laser energy absorption by C.sub.60. Furthermore, C.sub.60, is a non-linear optical material whose absorption increases by a factor of 3-5 for its excited states.
It is therefore an object of the present invention to provide a process for fabricating porous polycrystalline diamond material whose volume is described by a dimension larger than 1 mm, with no upper limit on size.
It is another object of the present invention to provide a process by which single pieces of diamond of arbitrary shape can be fabricated.
Heretofore diamond could only be produced in small size from natural diamond crystals discovered, or in thin films by CVD processes. By making a mold, filling it with diamond particles, and joining these particles by the C.sub.60 photofragmentation diamond growth processes, diamond of arbitrary shape can be produced, limited only by the necessary access for the optical excitation that fragments the C.sub.60 vapor permeating the part.
It is another object of the present invention to provide a process by which large pieces of diamond can be fabricated with controlled porosity.
By controlling the size and shape of the diamond particles used to form the bulk diamond the porosity of the final piece can be controlled. Larger particles will lead to larger pores, as will reduced compaction of a mass of smaller particles. Columnar passages can be created by first fabricating rods and then joining the rods together. A wide variety of internal geometries can be created using this and related concepts.
It is another object of the present invention to provide a process by which large pieces of diamond can be made with non-porous surfaces, having only internal porosity. Such pieces would be created by first making bulk porous pieces and then using conventional CVD diamond growth to add a solid, continuous diamond coating on the outside of these pieces.